Finished this afternoon! Loved it. Gathered all Green Stars and Stamps leading up to the post-game stuff (except for that fucking ninja house stamp...where is that shit!?).
Hint: It's in the last corridor, before you're transported to the roof. Spot the difference.
Answer:
Follow where the rolling spikes go (towards the camera). The give-away is supposed to be that they don't break when they hit the wall.
I had a really tough time with that one too. I'd say it's the hardest stamp in the entire game.